Steering wheels in general comprise a metal armature that is enclosed in an appropriate covering material such as wood, elastomeric substance, or a combination of the two. With the development of technology of attaching the steering wheel with the airbag module juxtaposed to the steering column (i.e. U.S. Pat. Nos. 5,897,132 and 5,692,769), there became an impetus for developing a single material composite steering wheel/airbag cover. The difficulty with this feat is the physical properties required of a material for the airbag are considerably different from those required of a material in the rim of a steering wheel. The obstacle of producing an integral steering wheel (rim, airbag cover, and spokes are molded from same material) has partially been overcome by utilizing reaction injection molded (RIM) polyurethane. This process produces a final surface that has very good wear properties while being compliant to the touch and relatively low in mass. These properties are highly desirable for a steering wheel rim and, in fact many standard (non-integrated) steering wheels are produced using this process. While this process has also been used extensively for airbag covers in the past, the use of RIM urethane is not nearly as desirable for this application due to the tendency of urethane to fragment during airbag deployment. U.S. Pat. No. 5,692,769 teaches that to overcome this drawback a tough, non-fragmenting material such as a substrate or a scrim is added beneath the airbag cover. Thus, it is inherent in this step that a substrate must be produced prior to the final urethane molding phase and must be introduced into the polyurethane molding tool, by some method, prior to introduction of the polyurethane. This step consumes time during the manufacturing process and adds complexity and cost to the molding due to the necessity of adding attachment features to secure the substrate during the RIM process.
The RIM polyurethane process has additional disadvantages: To promote proper filling of the mold cavity many air vents are employed and the tool is closed in such a way that some additional venting occurs at the meeting point of the two halves of the mold. Invariably some polyurethane material fills the vent areas during the molding process with the result that the molded part must later be trimmed to remove the excess material. During the trimming process it is easy to inadvertently damage the final surface of the product rendering it unacceptable for use.
An additional concern with the reaction injection molding process is the duration of the cycle. This cycle is typically two to three times longer in duration than the cycle associated with a standard injection molding process for a similar part.
To overcome the numerous disadvantages stated above it would be desirable to produce the final surface of the integrated steering wheel using an alternate process. The chief alternative used today to produce steering wheels and airbag covers is standard injection molding. Typically, automotive steering wheels are fabricated using thermoplastic polyvinyl chloride (PVC) based resins while airbag covers are fabricated from high-performance engineered resins known generally as thermoplastic elastomers. These two materials, as used in the subject applications, have very different physical properties. PVC is formulated for good wear characteristics and a targeted compressibility or "feel." The thermoplastic elastomers are chosen for chiefly their strength across a large temperature range and exhibit very little compliance. It is generally recognized that improving the compliance characteristics of a thermoplastic produces an adverse effect on strength and vice versa.